The invention relates to a compact two-group zoom optical system which has an angle of view of 70 or greater at the wide-angle end and satisfactory image-formation capability all over the zoom range while maintaining a zoom ratio of 3 or greater. A specific form of the invention employs a low frequency sonic-to-electrical transducer that monitors opening and closing of a disk-type steam trap. A two-dimensional correlational map is produced to have two parameters of an acceleration detected by the satellite sensor and speed variation derived from an acceleration detected by the floor G sensor.